Regenerative thermal oxidizers are conventionally used for destroying volatile organic compounds (VOCs) in high flow, low concentration emissions from industrial and power plants. Such oxidizers typically require high oxidation temperatures in order to achieve high VOC destruction. To achieve high heat recovery efficiency, the “dirty” process gas which is to be treated is preheated before oxidation. A heat exchanger column is typically provided to preheat these gases. The column is usually packed with a heat exchange material having good thermal and mechanical stability and sufficient thermal mass. In operation, the process gas is fed through a previously heated heat exchanger column, which, in turn, heats the process gas to a temperature approaching or attaining its VOC oxidation temperature. This pre-heated process gas is then directed into a combustion zone where any incomplete VOC oxidation is usually completed. The treated, now “clean”, gas is then directed out of the combustion zone and back through the heat exchanger column, or through a second heat exchange column. As the hot oxidized gas continues through this column, the gas transfers its heat to the heat exchange media in that column, cooling the gas and pre-heating the heat exchange media so that another batch of process gas may be preheated prior to the oxidation treatment. Usually, a regenerative thermal oxidizer has at least two heat exchanger columns which alternately receive process and treated gases. This process is continuously carried out, allowing a large volume of process gas to be efficiently treated.
The performance of a regenerative oxidizer may be optimized by increasing VOC destruction efficiency and by reducing operating and capital costs. The art of increasing VOC destruction efficiency has been addressed using, for example, means such as improved oxidation systems and purge systems (e.g., entrapment chambers), and three or more heat exchangers to handle the untreated volume of gas within the oxidizer during switch-over. Operating costs can be reduced by increasing the heat recovery efficiency, and by reducing the pressure drop across the oxidizer. Operating and capital costs may be reduced by properly designing the oxidizer and by selecting appropriate heat transfer packing materials.
An important element of an efficient oxidizer is the valving used to switch the flow of process gas from one heat exchange column or bed to another. Any leakage of untreated process gas through the valve system will decrease the efficiency of the apparatus. In addition, disturbances and fluctuations in the pressure and/or flow in the system can be caused during valve switch-over and are undesirable. Valve wear is also problematic, especially in view of the high frequency of valve switching in regenerative thermal oxidizer applications.
U.S. Pat. No. 6,261,092, the disclosure of which is hereby incorporated by reference, discloses a switching valve suitable for regenerative thermal oxidizers that addresses the foregoing issues. The drive system disclosed in the '092 patent is of the rack and pinion type with pneumatic cylinder actuation.
The rack and pinion pneumatically powered drive system has a number of limitations. First, it is a constant force design since the pneumatic air supply is typically regulated to a constant pressure. However, the conditions acting on the rotary switch valve will vary and result in varying force on the valve sealing surface. The resulting friction at the top sealing surface of the valve is a major factor in determining the force required to rotate the valve. Since this force is not constant, and the pneumatic cylinder actuation force is relatively constant, the performance of the drive system will vary. In some cases, it may not be able to rotate the valve. In other cases, it may rotate the valve too quickly and overcome the means to stop the valve.
A second limitation of the pneumatically actuated drive system is the minimal braking capacity of the pneumatic cylinder. Typically, only pneumatic air cushions or rubber cushions are available to absorb the energy of the moving drive and valve. Only small valves can be safely braked by these devices. Larger valves require that external energy absorbing devices such as shock absorbers be used to brake the valve. These devices increase the cost and reduce reliability of the drive.
A third limitation of the pneumatically actuated drive is that use in cold environments requires an extremely dry air supply to prevent ice blockage of the air line. This requires the addition of an expensive air drying device.
Finally, the rack and pinion pneumatic drive system has mechanical stops which are used to locate the valve accurately. However, failure of the valve braking device or rotating the valve too quickly and exceeding the capacity of the braking device may allow the drive and/or valve to become damaged by impact with the stops.
U.S. Pat. No. 7,308,904, the disclosure of which is hereby incorporated by reference, discloses a switching valve suitable for regenerative thermal oxidizers that addresses many of these issues. The drive system disclosed in the '904 patent includes a gear, gearbox, variable speed drive and at least one positioning sensor.
While this device has significant advantages over the prior art, there are several shortcomings. First, the vertical movement of the valve is provided using a pneumatic cylinder. This device requires compressed air to lift the valve. Such system may be susceptible to maintenance issues, especially in freezing temperatures. The range of vertical movement of the valve can also be somewhat limited when using a pneumatic cylinder.
Thus, it would be desirable to provide an alternative valve lift mechanism for a rotary valve, such as for the valve disclosed in the '092 and '904 patents, that allows for smooth and reliable operation, is cost effective, can lower the valve for servicing, can tolerate high and low temperatures and any expansion associated therewith, and is low maintenance. It also would be desirable to provide a regenerative thermal oxidizer that utilizes a valve equipped with such a valve lift mechanism.